FIGS. 7 to 11 show a conventional mechanical pencil.
As shown in FIG. 7, at a leading end area of a shaft cylinder 1, an outer periphery 11b of a chuck 11 at a position corresponding to a lead holding section is fitted into a fastener 12.
A chuck spring 7 is disposed between a shaft coupling 4 and a lead case 9 fixed to a rear end of the chuck 11. The chuck 11 is fastened by the fastener 12 by biasing force of the chuck spring 7, thereby holding the lead 10.
A holding chuck 8 made of resilient material such as rubber is fixed to a front portion of an inner hold of a front portion 3 threadedly mounted to a front end of the shaft cylinder 1. The lead 10 penetrates the holding chuck 8 and projects from the front portion 3.
From this state, the lead case 9 is knocked to move the chuck 11 forward, since the chuck 11 is fastened by the fastener 12, the lead 10 moves forward in a state where the lead 10 is held by the chuck 11.
If the lead 10 further moves forward, the fastener 12 abuts against an inner step 3a of the front portion 3 and its movement is stopped, and only the chuck 11 moves forward.
If the knock is released from this state, the chuck 11 is retracted in a state where the lead 10 is stopped by the holding chuck 8 (held), and the lead 10 is returned to a state shown in FIG. 7.
In a state where the fastener 12 is fitted to the chuck 11 from the retracted position again, the fastener 12 moves forward, and a front end of the fastener 12 abuts against an inner step 3a of the front portion 3. With this, the lead 10 is sent forward.
Next, a relation between the chuck 11 and the fastener 12 will be described in detail based on FIG. 8.
As shown in FIG. 8, an outer periphery 11b of the conventional chuck 11 corresponding to a lead holding section 11a is formed into an inclined surface 11c. The inclined surface is inclined gently through about 4° so as to obtain effective holding force. The conventional fastener 12 which is fitted over the outer periphery 11b of the chuck 11 is of cylindrical in shape, and its inner hole is a peripheral surface which is parallel to an axis of the inner hole.
Since the chuck 11 and the fastener 12 are formed in this manner, in a state where the lead 10 is held as shown in FIG. 8, the chuck 11 is fastened by the fastener 12 at the contact point 11d of the outer periphery of the chuck.
Here, a position where a line which is perpendicular to the inclined surface 11c of the chuck from the contact point 11d and an inner periphery of the lead holding section are intersecting with each other is a load application point 11e which functions as concentrated weight to the lead.
In the case shown in FIG. 8, in the length C (C=A+B) of the lead holding section 11a, the load application point 11e is at a position of A<B.
Next, a case in which a lead 10 having a slightly larger diameter is to be held will be explained based on FIG. 9.
In this case, in the length C (C=A+B) of the lead holding section 11a, the load application point 11e is at a position of A>B.
That is, in the case of the conventional chuck and fastener, since the inclination angle of the chuck is gentle, the position of the load application point 11e is largely varied due to a size error of an outer diameter of the chuck and due to variations of the diameter of the lead.
When a weight is applied to the lead in its axial direction by a writing pressure or the like, the chuck is further fastened, and the lead holding section further bites into the lead. The contact point 11d of the outer periphery of the chuck is deviated, and the load application point is further deviated as a result.
FIG. 10 is a transverse sectional view of the outer periphery 11b of the conventional chuck 11. A lead holding piece 11b will be explained based on FIG. 10. As shown in FIG. 10, the radius of curvature 11g of the lead holding section of the chuck 11 (lead holding piece 11b) is set slightly smaller than a radius 10a of the lead 10.
As a result, opposite angle portions of each of the lead holding pieces 11h divided by slots 11f bite into the lead 10, and the lead holding force is enhanced.
However, in the chuck 11 (lead holding piece 11b) shown in FIG. 10, both angle portions of the lead holding piece 11b bite into the lead 10, the lead is damaged, and when weight is repeatedly applied to the lead, there is a fear that biting-off (break) is caused.
As an improvement thereof, Japanese Patent Application Laid-open No. 2000-280683 (patent document 1) describes that if the radius of curvature of the lead holding section of the chuck is set equal to or slightly greater than a radius of a lead to be used, the lead can be used without breaking the same.
Further, as shown in FIG. 11, the lead holding section 11a of the conventional chuck 11 formed with fine thread projections and depressions 11i (height is about 50μ) is used. This has effective effect for increasing the lead holding force of the chuck 11.
However, this is not preferable because this damages the lead and the biting-off of the lead is caused.
Patent Document 1: Japanese Patent Application Laid-open No. 2000-280683